The invention relates to a fuel injection pump for internal combustion engines as disclosed hereinafter
In fuel injection it is advantageous for the supply quantity or injection quantity to be reduced and for the supply onset to begin later in the lower rpm range of the engine as compared with what is required at higher rpm. To this end, in a known fuel injection pump (Swiss Patent No. 269 597) a leakage connection between the pump work chamber and the inflow opening is provided which is effective for the first portion of the supply stroke that immediately follows the intake stroke of the pump piston. During this portion of the supply stroke, fuel flows back into the inflow opening via this leakage connection. The actual supply stroke of the pump piston, in which the aspirated fuel quantity is delivered under pressure to an injection nozzle, thus begins only after the leakage connection is closed. The supply and injection quantity is thus decreased in accordance with the returned fuel quantity. It is readily apparent that at low engine rpm and thus at a slow pump piston stroke speed, this effect is greater than at higher piston stroke speeds. In the upper rpm range, the leakage connection has virtually no effect.
In the known fuel injection pump, the leakage connection comprises an annular groove extending about the circumference of the piston and disposed in a manner that is spaced apart from and parallel to the piston end face defining the pump work chamber. The annular groove communicates with the pump work chamber via an axially extending leakage hole, so the leakage connection is closed only once the pump piston has moved so far upward that the annular groove is located with its upper control edge above the inflow opening in the cylinder bushing.
Because of the leakage hole, which penetrates the groove flank, the annular groove must be made relatively, deep. Since the annular groove must also be disposed relatively close to the end face of the piston, an outwardly projecting overhang or rim, which is not very thick, is accordingly formed on the outer rim of the piston end face that is exposed to high pressure; however, this rim cannot offer sufficient resistance, over a long period of time, to the pressure imposed on it. The result is a fuel injection pump with a short service life, because material fatigue sets in quickly.